176 The Bird 
are most numerous, but there are many exceptions. The 
bones of storks and vultures (birds of great powers of 
flight) are extremely pneumatic, while the bones of the 
flightless ostriches are filled with marrow, and in the 
aquatic penguins even this is reduced to a thread, the 
bones being almost wholly osseous tissue. A swan, 
although a heavy bird, flies remarkably well when once on 
the wing, and is highly aerated, but, on the other hand, 
terns and swifts—past-masters both in aerial evolutions 
—have solid bones! 
Now an athlete who is trained in running has always 
a very large lung capacity. Two persons of equal health 
and strength, one of whom has run many races or who 
has the power of keeping up a dog-trot for hour after hour, 
while the other has led a more sedentary life, may show 
a remarkable difference in the amount of air which they 
can draw into their lungs—perhaps one hundred or one 
hundred and fifty cubic inches more in the case of the 
runner. The average person uses only about one sev- 
enth of his lung capacity in ordinary breathing, the rest 
of the air remaining at the bottom of the lung, being 
termed ‘“residual.”’ As this is vitiated by its stay in the 
lung, it does harm rather than good by its presence. When 
great exertion is required, as in running, the person who 
can admit the largest amount of fresh air to his lungs in 
each breath has command of an equally great power 
of action. 
As we have seen, the lungs of a bird are small and 
non-elastic, but this is more than compensated by the 
continuous passage of fresh air, passing not only into 
